Simplify the following expression: $x = \dfrac{1}{2r} - \dfrac{8}{5r}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $2r$ and $5r$ $\lcm(2r, 5r) = 10r$ $ x = \dfrac{5}{5} \cdot \dfrac{1}{2r} - \dfrac{2}{2} \cdot \dfrac{8}{5r} $ $x = \dfrac{5}{10r} - \dfrac{16}{10r}$ $x = \dfrac{5 -16}{10r}$ $x = \dfrac{-11}{10r}$